1. Prospects for Future measurement of GPDs using COMPASS at CERN
E. Burtin, CEA/Saclay - Dapnia/SPhN
for the COMPASS Collaboration
Expression of Interest SPSC-EOI-2005
Physics motivations & context
Experimental realisation
INPC 2007
Tokyo, June 5, 2007

2. Generalized Parton Distributions
GPDs
x+
x- P P’
hard
soft g*
g,p,r t
Factorisation:
Q2 large, -t<1 GeV2
contains pdf
H(x,0,0) = q(x)
measured in DIS
Generalized Parton Distributions
for quarks :
4 functions H,E,H,E(x,,t)
~~
contains form factors
F(t)
measured in elastic scattering
contains information on the nucleon spin :
Ji’s sum rule :
and 1/2 = 1/2  + Lz + G = Jz + G

3. Towards a 3-D nucleon picture (Px,ry,z)
Lattice calculation (unquenched QCD):
Negele et al., NP B128 (2004) 170
Göckeler et al., NP B140 (2005) 399
 small valence quark core
 widely spread sea q and gluons
Hard Exclusive Scattering
Deeply Virtual Compton Scattering
ep ep
GPDs * Q²
x+
x- p  t
mp=0.87 GeV p z
x P r y
x boost
measure H(x,0,t) as function of t
Chiral dynamics:
Strikman et al., PRD69 (2004)
gluon density generated by the  cloud
increase of the N transverse size
for xBj < mπ/mp=0.14
GPDs : H( x,,t )
Fourier
access to correlations :
( Px, ry,z )
Burkardt,Belitsky,Müller,Ralston,Pire

4. DVCS observables Deep VCS Bethe-Heitler High energy beam
Lower energy => use interference - holography
Cross section
Single Spin Difference
Polarised beam
Beam Charge Difference
+/- charged beam

5. What makes Compass a special place ?
CERN High energy muon beam
100 / 190 GeV
80% Polarisation
+q and -q available (change once/day)
Same intensity & scrapper settings
Opposite polarization
Kinematical domain :
Hera
Ix2
E=190,
100GeV
Targets
liquid Hydrogen & Deuterium
Polarised target (no recoil)
Calendar
Now : COMPASS with polarised target
: Extensive program with
Recoil detection & Calo
2014+ : Jlab 12 and GSI/FAIR startup
Valence + sea + gluons

6. Projected errors of a possible DVCS experiment
Beam Charge Asymmetry
Ebeam = 100 GeV
180 days, 25 % efficiency
L = cm-2 s-1
6/18 (x,Q²) data samples
Double Distribution : VGG
PRD60(1999) and Prog.Part. NP47(2001)
Factorized t-dependence
H(x,ξ,t) ~ q(x) F(t)
Reggeized t-dependence
<b2>=’ln1/x (’=0.8,1.1)
H(x,0,t) = q(x) et<b2>= q(x)/xα’t
Dual parametrization
Mellin moments, QCD evolution
V. Guzey PRD74(2006)054027
Good constrains for models

7. Contribution of Ju and Jd to the nucleon spin
Through the modeling of GPD E in Double Distribution formalism
1-Transversaly polarised target and unpolarized beam
In Meson production :
Under study with COMPASS LiD deuteron Data
and with 2007 NH3 proton Data
In DVCS :
But… no recoil detection around the polarized target
2-Neutron target - liquid deuterium
for the complete program after 2010

8. Experimental realisation
all COMPASS trackers:
SciFi, Si, μΩ, Gem, DC, Straw, MWPC
2.5 m Liquid H2 & D target
to be designed and built μ’
ECAL 1 or 2
  10° 
COMPASS equipment
with additional calorimetry
at large angle (p0 bkg) μ p’
Recoil detector to insure exclusivity
to be designed and built
TOF~300ps

9. Recoil Detector Prototype of scint. counters:
B: 4m x 5cm A: 2.8m x 0.4cm
TOF = 315 ps achieved (MIP)
Using 1GHz sampler and waveform
Analysis (mandatory for the expt)

10. GEANT Simulation of PMT signals with beam
Red is DVCS proton
Blue is background
upstream
PMT
downstream
INNER
OUTER
Proton efficiency
Effective signal
Beam intensity (muons/5s)
recording the
waveform of all
signals and segmentation are mandatory

11. Calorimeter acceptance
xbj Q2
Existing Calorimeters
Xbj-bins
+ 3m x 3m ECAL0

12. Physical Background to DVCS
Source : Pythia 6.1 generated DIS events
Apply DVCS-like cuts
’,,p in DVCS range
no other charged & neutral in active volumes
detector requirements:
24° coverage for neutral
40° for charged particles
in this case
DVCS is dominant

13. Proton signal with the prototype
deuterons
protons
Measured β
E in B (MeV)
Target : 10 cm CH
Nominal beam intensity
8 hours of data

14. Conclusion & prospects
Possible physics ouput
Sensitivity to spatial distribution of partons
Sensitivity to total spin of partons : Ju & Jd
Working on a variety of models (VGG, Müller, Guzey and FFS-Sch) to quantify the Physics potential of DVCS at COMPASS
Experimental realisation
DVCS proton is accompanied by a high background
Recoil Detection is feasible (waveforms DAQ)
Extension of the calorimetry is desirable
Roadmap
“exclusive” meson production on transverse target results soon…
Building a 1m long Recoil Detector for the COMPASS hadron program
Submission of a proposal as soon as possible
Get ready to take data with recoil detection and extended calorimetry from 2010 to 2015 before JLab12 & GSI/Fair